Current monitoring devices for AC electric power systems typically employ current transformers for providing input currents that are isolated from the conductors of the electric power system. For example, referring to the conventional current transformer CT1 shown in FIG. 1, a conductor 1 of a power system is configured as a primary winding of the current transformer CT1 and extends through a toroid magnetic core 2. The term “magnetic core” as used herein refers to a magnetic body having a defined relationship with one or more conductive windings. A secondary winding 3 is magnetically coupled to the magnetic core 2. The phrase “magnetically coupled” is defined herein to mean that flux changes in the magnetic core 2 are associated with an induced voltage in the secondary winding 3, wherein the induced voltage is proportional to the rate of change of magnetic flux in accordance with Faraday's Law.
Current flowing through the primary winding 1 and passing through the magnetic field of the magnetic core 2 induces a secondary current in the secondary winding 3, wherein the magnitude of the secondary current corresponds to a ratio (commonly referred to as the “CT ratio”) of the number of turns in the primary and secondary windings 1 and 3. The primary winding 1 may include only one turn (as in FIG. 1) or may include multiple turns wrapped around the magnetic core 2. The secondary winding typically includes multiple turns wrapped around the magnetic core 2. The secondary winding 2 is connected to a protection relay (not shown) that measures the induced secondary current. The protection relay uses this measured current to provide overcurrent protection and metering functions.
Traditionally, protection relays and associated current transformers have been designed for electrical power systems that operate at fixed frequencies (e.g., 50/60 Hz). However, with the recent increase in the use of variable-frequency drives for controlling the operation of electric motors, there is a need for protection relays that employ current transformers that are capable of detecting both AC and DC faults.
FIG. 2 illustrates a prior art differential current sensor 10 that can detect AC and DC components of a differential current by utilizing an oscillating circuit. In particular, a summation current converter comprises two oppositely applied windings W1 and W2 having the same number of turns wound about a magnetic core M. During operation, the switches S1 and S2 of an oscillator are opened and closed in an alternating fashion so that the windings W1 and W2 carry current in alternation. The oscillating circuit changes state when the magnetic core M becomes saturated by the current in the windings W1 and W2. Upon saturation of the magnetic core M, there is no change in the current flowing through the current carrying winding W1 or W2, as the inductance of the winding W1 or W2 becomes negligibly slight so that no voltage can be induced at the control input of the switch S1 or S2 that has been closed, either. The switch S1 or S2 therefore opens. The opening of the switch S1 or S2 causes the voltage Ub (fixed direct supply voltage) to appear at the control input, and a corresponding induction voltage of the non-conducting winding W1 or W2 is formed. The previously opened switch S1 or S2 thereupon closes.
Because the switches S1 and S2 close in alternation, the current flow through the current sensor 10 results in a voltage drop at the measuring resistors Rm, which operate at frequencies that correspond to the oscillation frequency. By determining the difference between the voltage drops across the resistors Rm, the two branches of the oscillator can be evaluated. The differential voltage Udif can be considered to be a square wave voltage, thus facilitating recovery of the AC and DC components of the differential current therefrom.
While prior art AC/DC current sensors such as the one described above are generally effective for their intended purpose, they can be expensive. It would therefore be advantageous to provide a current sensor that is capable of detecting both AC and DC faults and that is relatively inexpensive.